1. Field of the Invention
The present invention relates to a focus monitoring method in an exposure apparatus used for lithography of a semiconductor device and, more particularly, to a focus monitoring method of obtaining an optimal focus position of an EUV exposure apparatus using, e.g., soft X-rays.
2. Description of the Related Art
In recent years, importance is attached to the throughput in a lithography process for manufacturing, e.g., a semiconductor device or liquid crystal display device. From this viewpoint, there is often used an exposure apparatus of a sequential movement type such as a reduction projection exposure apparatus (so-called stepper) of a step & repeat scheme or a scanning exposure apparatus of a step & scan scheme as an improved version of the stepper. An exposure apparatus of this type mainly uses a so-called CD/focus method and SMP focus measurement method to measure an optimal focus position (best focus position) of an optical system (see, e.g., Japanese Patent Nos. 2580668 and 2712330).
The CD/focus method transfers a test pattern such as a line-and-space pattern to a plurality of positions on a test wafer in the optical axis direction of the projection optical system. A scanning electron microscope (SEM), for example, measures the line width of a resist image (transferred pattern image) obtained by developing the test wafer. The best focus position is then determined on the basis of the correlation between the measured line width and the wafer positions (to be also referred to as “focus positions” hereinafter as needed) in the optical axis direction of the projection optical system.
The SMP focus measurement method forms a resist image with a wedge-shaped mark at each of a plurality of focus positions. A change in the line width of the resist image between the focus positions is amplified and translated into a change in the dimension of the resist image in the longitudinal direction. The dimension of the resist image in the longitudinal direction is measured using a mark detection system such as an alignment system of the exposure apparatus. The best focus position is then detected (calculated) on the basis of the correlation between the focus position and the length of the resist image. On the basis of the thus obtained best focus position, astigmatism, curvature of field, and the like as the optical characteristics of the projection optical system are measured for various kinds of test patterns.
However, the CD/focus method requires strict focus adjustment of the SEM to measure the line width of, e.g., a resist image, resulting in too long a measurement time per point. Accordingly, it takes several hours to several tens of hours to perform measurement at a large number of points. Along with the micronization of a test pattern to measure the optical characteristics of the projection optical system, the number of evaluation points in the field of view of the projection optical system is expected to increase. Therefore, the conventional measurement method using an SEM inconveniently, considerably decreases the throughput until the measurement result is obtained.
It is becoming difficult for the conventional measurement method to meet growing demands for further reducing a measurement error and further increasing the reproducibility of the measurement result. To reduce the error, an approximation curve of the fourth or higher order is used to describe the correlation between the focus position and the line width. This requires calculating the line width at least at five focus positions for each evaluation point. To reduce the error, it is also necessary that the difference between the line width at the best focus position and that at a focus position (including both the positive and negative directions of the optical axis of the projection optical system) shifted from the best focus position should be 10% or more. However, it is becoming difficult to meet this condition.
The SMP focus measurement method normally performs measurement with monochromatic light, so interference may have a different influence on the resist image depending on its shape, leading to a measurement error (dimensional offset). To measure the length of a resist image with a wedge-shaped mark in image processing, it is necessary to capture detailed information ranging across the two ends of the resist image in the longitudinal direction, at which its pattern width is finest. However, the currently available image capturing devices (e.g., a CCD camera) have not obtained sufficient resolutions yet. It is also difficult to increase the number of evaluation points in the field of view of the projection optical system because the test pattern is large.
In this manner, the conventional EUV exposure apparatus using soft X-rays can hardly calculate an accurate optimal focus position of the optical system. Still worse, the measurement requires a relatively long time, resulting in decreases in exposure accuracy and throughput.